Abstract:
Many hydrocarbons reservoirs are capped by thick layers of rocksalt, a material that creeps when a shear stress is applied to it. Pumping leads to compaction of the reservoir material but also induces shear stresses inside rocksalt. These need to relax, leading to time-dependent ground deformations and surface subsidence. To quantify creep-induced subsidence we have created simplified yet realistic Finite Element models of medium-sized rocksalt-capped reservoirs that include key aspects of the subsurface geometry and material properties layering. These show that creep-induced subsidence can be a significant proportion of the total subsidence and so rocksalt caprocks need to be carefully modeled. We discuss uncertainties relating to the low-stress creep behavior of rocksalt and show the subsidence evolution predicted by the use of different constitutive models for rocksalt creep. Another source of uncertainty is the poor characterization of the rocksalt body's physical properties spatial distribution. Here we touch upon rocksalt grain-size and chemical composition as two such major uncertainties and discuss the effects this can have on the overall subsidence response. We finish with the conclusion that engineers who are faced with the task of predicting the subsidence evolution have to resort to calculating bounds for the resultant response based on a number of possible scenaria for rocksalt body behavior.
Introduction
A large number of economically important hydrocarbons reservoirs worldwide are overlain by thick rocksalt layers. Examples include the natural gas fields in the Netherlands, the large hydrocarbons fields of Brazil (e.g. Mohriak et al., 2012), or of the Gulf of Mexico (e.g. Dribus et al., 2008), and the hydrocarbon fields of northwest China (Yu et al., 2014). Time-dependent subsidence above reservoirs (or stated in a slightly different way - subsidence that is not proportional to the volumes of extracted hydrocarbons) seems to be a widespread phenomenon (e.g. Hettema et al., 2002). The mechanisms through which time-dependent subsidence develops can be numerous, and can include reservoir inelasticity and creep, delayed compaction due to delayed pore pressure equilibration inside a reservoir system.
With previous work we have shown that for rocksalt-capped reservoirs, subsidence due to rocksalt creep can be significant (see Marketos et al., 2015). The above observation alone requires that special focus is given on modeling the rocksalt material response, especially for low-lying coastal regions where subsidence calculations need to be very accurate. With this paper we first briefly present how rocksalt flow can give rise to time-dependent subsidence and then discuss a number of uncertainties related to the rocksalt. With the aid of a Finite Element model of a rocksalt-capped reservoir we then focus on the effects of these uncertainties on subsidence calculations and propose a way forward in order to practically deal with these uncertainties.
